The present invention relates to the use of ytterbium(III) acetylacetonate as a catalyst for the preparation of aliphatic oligocarbonate polyols by transesterification of an organic carbonate with an aliphatic polyol, to polyols produced using such catalyst and to prepolymers produced from such polyols.
Oligocarbonate polyols are important raw materials, for example, in the production of plastics materials, coatings and adhesives. They are reacted, for example, with isocyanates, epoxides, (cyclic) esters, acids or acid anhydrides (DE-A 1 955 902). They can in principle be prepared from an aliphatic polyol by reaction with phosgene (for example, DE-A 1 595 446), bis-chlorocarbonic ester (for example, DE-A 857 948), diaryl carbonate (for example, DE-A 1 01 2557), cyclic carbonate (for example, DE-A 2 523 352) or dialkyl carbonate (for example, WO 2003/2630).
It is known that when aryl carbonates such as diphenyl carbonate are reacted with aliphatic polyols such as 1,6-hexanediol a satisfactory reaction conversion can be achieved solely by removing the liberated alcoholic compound (for example, phenol) during the course of the equilibrium shift of the reaction (for example, EP-A 0 533 275). However, if alkyl carbonates (for example, dimethyl carbonate) are used, transesterification catalysts such as alkali metals or alkaline earth metals as well as oxides, alkoxides, carbonates, borates thereof or salts of organic acids (for example, WO 2003/2630) are frequently used.
Moreover, tin or organotin compounds such as bis(tributyltin) oxide, dibutyltin laurate or alternatively dibutyltin oxide (DE-A 2 523 352) as well as compounds of titanium such as titanium tetrabutylate, titanium tetraisopropylate or titanium dioxide are preferably used as transesterification catalysts (for example, EP-B 0 343 572 and WO 2003/2630).
The transesterification catalysts known from the prior art for the preparation of aliphatic oligocarbonate polyols by reacting alkyl carbonates with aliphatic polyols, however, have some disadvantages.
Organotin compounds have recently been recognized as potential human carcinogens. They are consequently undesirable constituents and previously preferred catalyst compounds such as bis(tributyltin) oxide, dibutyltin oxide or dibutyltin laurate persist in secondary products of the oligocarbonate polyols. When strong bases such as alkali metals or alkaline earth metals or alkoxides thereof are used, it is necessary to neutralize the products in an additional process step after completion of the oligomerization. If, on the other hand, Ti compounds are used as catalysts, undesirable discoloration (yellowing) of the resulting product may occur during storage, which is brought about by the presence of Ti(III) compounds alongside Ti(IV) compounds and/or by the tendency of titanium to form complexes.
In addition to this undesirable characteristic of discoloration, when the hydroxyl-terminating oligocarbonates are reacted further as a raw material in the production of a polyurethane, titanium-containing catalysts have high activity vis-à-vis compounds which contain isocyanate groups. This characteristic is particularly conspicuous when the titanium-catalyzed oligocarbonate polyols are reacted with aromatic (poly)isocyanates at elevated temperature, such as is the case, for example, in the production of pouring elastomers or thermoplastic polyurethanes (TPUs). This disadvantage can even result in shortening of the pot life or reaction time of the reaction mixture as a result of utilisation of titanium-containing oligocarbonate polyols, to such an extent that it is no longer possible to use such oligocarbonate polyols for these fields of application. In order to avoid this disadvantage, the transesterification catalyst which persists in the product is as far as possible inactivated in at least one additional production step once the synthesis is concluded.
EP-B 1 091 993 teaches inactivation by the addition of phosphoric acid, whereas U.S. Pat. No. 4,891,421 proposes inactivation by hydrolysis of the titanium compounds, with a corresponding quantity of water being added to the product and being removed again from the product by distillation once deactivation has been achieved.
It has not furthermore been possible with the catalysts used hitherto to reduce the reaction temperature, which is normally between 150° C. and 230° C., in order largely to avoid the formation of by-products such as ethers or vinyl groups, which may arise at elevated temperature. These undesirable terminal groups act as chain terminators for subsequent polymerization reactions. For example, in the case of the polyurethane reaction with polyfunctional (poly)isocyanates, they lead to a lowering of the network density and hence to poorer product characteristics (for example, solvent or acid resistance).
Moreover, oligocarbonate polyols which have been prepared with the aid of the catalysts known from the prior art have high ether group (for example, methylether, hexylether, etc.) contents. However, these ether groups in the oligocarbonate polyols lead, for example, to unsatisfactory hot air resistance of pouring elastomers which are based on such oligocarbonate polyols, because ether compounds in the material are broken down under these conditions, thus leading to material failure.